A deflection yoke to deflect or accelerate an electron beam in, e.g., a display monitor device and a flyback transformer are known as magnetic field generating apparatus. A leakage magnetic field generated from such a magnetic field generating apparatus needs to be cancelled in order to avoid adverse influences on the other parts and apparatus.
FIG. 1 is a side elevational view showing a conventional leakage magnetic field cancelling apparatus in which a molded portion is removed for simplicity of the drawing. In the diagram, reference numeral 1 denotes a flyback transformer as a magnetic field generating apparatus; 2 is a coil of the flyback transformer 1; 3 a bobbin of the transformer 1; and 4 a leakage magnetic field which is eventually generated from the transformer 1.
A shielding ring 5 is formed of an aluminum or copper plate and generates a magnetic field 6 to cancel the leakage magnetic field 4 from the transformer 1. In general, the ring 5 is formed by an aluminum plate for the purpose of reduction of the weight and is used to shield the electromagnetic waves and electrostatic charges.
The shielding ring 5 is clamped by a clamper or the like (not shown) which is attached to a heat radiating plate or base plate (not shown).
The operation will now be described.
When a current is supplied to the coil 2 of the flyback transformer 1 in order to accelerate an electron beam, the leakage magnetic field 4 is generated so as to pass through the outside of the shielding ring 5. The leakage magnetic field 4 is formed by a part of magnetic line of force generated by the coil 2.
The magnetic field 6 in the opposite direction which is proportional to the leakage magnetic field 4 is generated from the shielding ring 5 by the leakage magnetic field 4 and at the same time, the current flowing through the ring 5 is radiated as the heat by the leakage magnetic field 4.
Since the whole of the magnetic field 6 generated from the ring 5 passes through the outside of the ring 5 in this manner, the influence of the leakage magnetic field 4 can be cancelled by the magnetic field 6. Namely, it is possible to reduce the distortion on the screen of a CRT (cathode ray tube) which is caused by the leakage magnetic field 4 for the deflection yoke of the flyback transformer.
Since the conventional leakage magnetic field cancelling apparatus is constituted in this manner, there is a danger of breakdown of the base plate by the heavy shielding ring 5.
In addition, since the ring 5 is formed by an aluminum or copper plate, the ring 5 is expensive, so that there are problems such that the leakage magnetic field cancelling apparatus becomes expensive and only the leakage magnetic field 4 can be cancelled.